Wireless access system using multiple modulation formats in TDD frames and method of operation

ABSTRACT

There is disclosed a radio frequency (RF) modem shelf for use in a fixed wireless access network comprising a plurality of base stations capable of bidirectional time division duplex (TDD) communication with wireless access devices disposed at a plurality of subscriber premises. The radio frequency (RF) modem shelf comprises: a) a first RF modem for communicating with a plurality of the wireless access devices using TDD frames, each TDD frame having an uplink for receiving data and a downlink for transmitting data; and b) a modulation controller associated with the RF modem shelf for determining an optimum modulation configuration for each of the plurality of wireless access devices communicating with the first RF modem. The modulation controller causes the first RF modem to transmit downlink data to a first wireless access device in a first data block having a first optimum modulation configuration and to transmit downlink data to a second wireless access device in a second data block having a different second optimum modulation configuration.

BACKGROUND OF THE INVENTION

[0001] Telecommunications access systems provide for voice, data, andmultimedia transport and control between the central office (CO) of thetelecommunications service provider and the subscriber (customer)premises. Prior to the mid-1970s, the subscriber was provided phonelines (e.g., voice frequency (VF) pairs) directly from the Class 5switching equipment located in the central office of the telephonecompany. In the late 1970s, digital loop carrier (DLC) equipment wasadded to the telecommunications access architecture. The DLC equipmentprovided an analog phone interface, voice CODEC, digital datamultiplexing, transmission interface, and control and alarm remotelyfrom the central office to cabinets located within business andresidential locations for approximately 100 to 2000 phone lineinterfaces. This distributed access architecture greatly reduced linelengths to the subscriber and resulted in significant savings in bothwire installation and maintenance. The reduced line lengths alsoimproved communication performance on the line provided to thesubscriber.

[0002] By the late 1980s, the limitations of data modem connections overvoice frequency (VF) pairs were becoming obvious to both subscribers andtelecommunications service providers. ISDN (Integrated Services DigitalNetwork) was introduced to provide universal 128 kbps service in theaccess network. The subscriber interface is based on 64 kbpsdigitization of the VF pair for digital multiplexing into high speeddigital transmission streams (e.g., T1/T3 lines in North America, E1/E3lines in Europe). ISDN was a logical extension of the digital networkthat had evolved throughout the 1980s. The rollout of ISDN in Europe washighly successful. However, the rollout in the United States was notsuccessful, due in part to artificially high tariff costs which greatlyinhibited the acceptance of ISDN.

[0003] More recently, the explosion of the Internet and deregulation ofthe telecommunications industry have brought about a broadbandrevolution characterized by greatly increased demands for both voice anddata services and greatly reduced costs due to technological innovationand intense competition in the telecommunications marketplace. To meetthese demands, high speed DSL (digital subscriber line) modems and cablemodems have been developed and introduced. The DLC architecture wasextended to provide remote distributed deployment at the neighborhoodcabinet level using DSL access multiplexer (DSLAM) equipment. Theincreased data rates provided to the subscriber resulted in upgradeDLC/DSLAM transmission interfaces from T1/E1 interfaces (1.5/2.0 Mbps)to high speed DS3 and OC3 interfaces. In a similar fashion, the entiretelecommunications network backbone has undergone and is undergoingcontinuous upgrade to wideband optical transmission and switchingequipment.

[0004] Similarly, wireless access systems have been developed anddeployed to provide broadband access to both commercial and residentialsubscriber premises. Initially, the market for wireless access systemswas driven by rural radiotelephony deployed solely to meet the universalservice requirements imposed by government (i.e., the local telephonecompany is required to serve all subscribers regardless of the cost toinstall service). The cost of providing a wired connection to a smallpercentage of rural subscribers was high enough to justify thedevelopment and expense of small-capacity wireless local loop (WLL)systems.

[0005] Deregulation of the local telephone market in the United States(e.g., Telecommunications Act of 1996) and in other countries shiftedthe focus of fixed wireless access (FWA) systems deployment from ruralaccess to competitive local access in more urbanized areas. In addition,the age and inaccessibility of much of the older wired telephoneinfrastructure makes FWA systems a cost-effective alternative toinstalling new, wired infrastructure. Also, it is more economicallyfeasible to install FWA systems in developing countries where the marketpenetration is limited (i.e., the number and density of users who canafford to pay for services is limited to small percent of thepopulation) and the rollout of wired infrastructure cannot be performedprofitably. In either case, broad acceptance of FWA systems requiresthat the voice and data quality of FWA systems must meet or exceed theperformance of wired infrastructure.

[0006] Wireless access systems must address a number of uniqueoperational and technical issues including:

[0007] 1) Relatively high bit error rates (BER) compared to wire line oroptical systems; and

[0008] 2) Transparent operation with network protocols and protocol timeconstraints for the following protocols:

[0009] a) ATM;

[0010] b) Class 5 switch interfaces (domestic GR-303 and internationalV5.2);

[0011] c) TCP/IP with quality-of-service QoS for voice over IP (VoIP)(i.e., RTP) and other H.323 media services;

[0012] d) Distribution of synchronization of network time out to thesubscribers;

[0013] 3) Increased use of voice, video and/or media compression andconcentration of active traffic over the air interface to conservebandwidth;

[0014] 4) Switching and routing within the access system to distributesignals from the central office to multiple remote cell sites containingmultiple cell sectors and one or more frequencies of operation persector; and

[0015] 5) Remote support and debugging of the subscriber equipment,including remote software upgrade and provisioning.

[0016] Unlike physical optical or wire systems that operate at bit errorrates (BER) of 10⁻¹¹, wireless access systems have time varying channelsthat typically provide bit error rates of 10⁻³ to 10⁻⁶. The wirelessphysical (PHY) layer interface and the media access control (MAC) layerinterface must provide modulation, error correction and ARQ (automaticrequest for retransmission) protocol that can detect and, whererequired, correct or retransmit corrupted data so that the interfaces atthe network and at the subscriber site operate at wire line bit errorrates.

[0017] The wide range of equipment and technology capable of providingeither wireline (i.e., cable, DSL, optical) broadband access or wirelessbroadband access has allowed service providers to match the needs of asubscriber with a suitable broadband access solution. However, in manyareas, the cost of cable modem or DSL service is high. Additionally,data rates may be slow or coverage incomplete due to line lengths. Inthese areas and in areas where the high cost of replacing old telephoneequipment or the low density of subscribers makes it economicallyunfeasible to introduce either DSL or cable modem broadband access,fixed wireless broadband systems offer a viable alternative. Fixedwireless broadband systems use a group of transceiver base stations tocover a region in the same manner as the base stations of a cellularphone system. The base stations of a fixed wireless broadband systemtransmit forward channel (i.e., downstream) signals in directed beams tofixed location antennas attached to the residences or offices ofsubscribers. The base stations also receive reverse channel (i.e.,upstream) signals transmitted by the broadband access equipment of thesubscriber.

[0018] Media access control (MAC) protocols refer to techniques thatincrease utilization of two-way communication channel resources bysubscribers that use the channel resources. The MAC layer may use anumber of possible configurations to allow multiple access.

[0019] These configurations include:

[0020] 1. FDMA—frequency division multiple access. In a FDMA system,subscribers use separate frequency channels on a permanent or demandaccess basis.

[0021] 2. TDMA—time division multiple access. In a TDMA system,subscribers share a frequency channel but allocate spans of time todifferent users.

[0022] 3. CDMA—code division multiple access. In a CDMA system,subscribers share a frequency but use a set of orthogonal codes to allowmultiple access.

[0023] 4. SDMA—space division multiple access—In a SDMA system,subscribers share a frequency but one or more physical channels areformed using antenna beam forming techniques.

[0024] 5. PDMA—polarization division multiple access—In a PDMA system,subscribers share a frequency but change polarization of the antenna.

[0025] Each of these MAC techniques makes use of a fundamental degree offreedom (physical property) of a communications channel. In practice,combinations of these degrees of freedom are often used. As an example,cellular systems use a combination of FDMA and either TDMA or CDMA tosupport a number of users in a cell.

[0026] To provide a subscriber with bi-directional (two-way)communication in a shared media, such as a coaxial cable, a multi-modefiber (optical), or an RF radio channel, some type of duplexingtechnique must be implemented. Duplexing techniques include frequencydivision duplexing (FDD) and time division duplexing (TDD). In FDD, afirst channel (frequency) is used for transmission and a second channel(frequency) is used for reception. To avoid physical interferencebetween the transmit and receive channels, the frequencies must have aseparation know as the duplex spacing. In TDD, a single channel is usedfor transmission and reception and specific periods of time (i.e.,slots) are allocated for transmission and other specific periods of timeare allocated for reception.

[0027] Finally, a method of coordinating the use of bandwidth must beestablished. There are two fundamental methods: distributed control andcentralized control. In distributed control, subscribers have a sharedcapability with or without a method to establish priority. An example ofthis is CSMA (carrier sense multiple access) used in IEEE802.3 Ethernetand IEEE 802.11 Wireless LAN. In centralized control, subscribers areallowed access under the control of a master controller. Cellularsystems, such as IS-95, IS-136, and GSM, are typical examples. Access isgranted using forms of polling and reservation (based on polled ordemand access contention).

[0028] A number of references and overviews of demand access areavailable including the following:

[0029] 1. Sklar, Bernard. “Digital Communications Fundamentals andApplications,” Prentice Hall, Englewood Cliffs, N.J., 1988. Chapter 9.

[0030] 2. Rappaport, Theodore. “Wireless Communications, Principles andPractice,” Prentice Hall, Upper Saddle River, N.J., 1996. Chapter 8.

[0031] 3. TR101-173V1.1. “Broadband Radio Access Networks, Inventory ofBroadband Radio Technologies and Techniques,” ETSI, 1998. Chapter 7.

[0032] The foregoing references are hereby incorporated by referenceinto the present disclosure as if fully set forth herein.

[0033] In 1971, the University of Hawaii began operation of a randomaccess shared channel ALOHA TDD system. The lack of channel coordinationresulted in poor utilization of the channel. This lead to theintroduction of time slots (slotted Aloha) that set a level ofcoordination between the subscribers that doubled the channelthroughput. Finally, the researchers introduced the concept of a centralcontroller and the use of reservations (reservation Aloha). Reservationtechniques made it possible to make trade-offs between throughput andlatency.

[0034] This work was fundamental to the development of media accesscontrol (MAC) techniques for dynamic random access and the use of ARQ(automatic request for retransmission) to retransmit erroneous packets.While the work at the University of Hawaii explored the fundamentals ofburst transmission and random access, the work did not introduce theconcept of a frame and/or super-frame structure to the TDD/TDMA accesstechniques. One of the more sophisticated systems developed in the 1970sand in current use is Joint Tactical Information Distribution System(JTIDS). This system was based on the joint use of TDMA and timeduplexing over frequency-hopping spread-spectrum channels. This was theculmination of research to allow flexible allocation of bandwidth to alarge group of users. The key aspect of the JTIDS system was theintroduction of dynamic allocation of bandwidth resources and explicitvariable symmetry (downlink vs. uplink bandwidth) in the link.

[0035] IEEE 802.11 Wireless LAN equipment provides for a centrallycoordinated TDD system that does not have a specific frame or slottingstructure. IEEE 802.11 did introduce the concept of variable modulationand spreading inherent in the structure of the transmission bursts. Asignificant improvement was incorporated in U.S. Pat. No. 6,052,408,entitled “Cellular Communications System with Dynamically Modified DataTransmission Parameters.” This patent introduced specific burst packettransmission formats that provide for adaptive modulation, transmitpower, and antenna beam forming and an associated method of determiningthe highest data rate for a defined error rate floor for the linkbetween the base station and a plurality of subscribers assigned to thatbase station. With the exception of variable spreading military systemsand NASA space communication systems, this was one of the firstcommercial patents that address variable transmission parameters toincrease system throughput.

[0036] Another example of TDD systems are digital cordless phones, alsoreferred to as low-tier PCS systems. The Personal Access Communications(PAC) system and Digital European Cordless Telephone (DECT, as specifiedby ETSI document EN 300-175-3) are two examples of these systems.Digital cordless phones met with limited success for their intended useas pico-cellular fixed access products. The systems were subsequentlymodified and repackaged for wireless local loop (WLL) applications withextended range using increased transmission (TX) power and greaterantenna gain.

[0037] These TDD/TDMA systems use fixed symmetry and bandwidth betweenthe uplink and the downlink. The TDD frame consists of a fixed set oftime slots for the uplink and the downlink. The modulation index (ortype) and the forward error correction (FEC) format for all datatransmissions are fixed in these systems.

[0038] These systems did not include methods for coordinating TDD burstsbetween systems. This resulted in inefficient use of spectrum in thefrequency planning of cells.

[0039] While DECT and PAC systems based on fixed frames with fixed andsymmetric allocation of time slots (or bandwidth) provides excellentlatency and low jitter, and can support time bounded services, such asvoice and Nx64 Kbps video, these systems do not provide efficient use ofthe spectrum when asymmetric data services are used. This has lead toresearch and development of packet based TDD systems based on Internetprotocol (IP) or asynchronous transfer mode (ATM), with dynamicallocation of TDD time slots and the uplink-downlink bandwidth, combinedwith efficient algorithms to address both best efforts and real-timelow-latency service for converged media access (data and multi-media).

[0040] One example of a TDD system with dynamic slot and bandwidthassignment is the ETSI HYPERLAN II specification based on the DynamicSlot Assignment algorithm described in “Wireless ATM: PerformanceEvaluation of a DSA++ MAC Protocol with Fast Collision resolution byProbing Algorithm,” D. Petras and A. Kramling, International Journal ofWireless Information Networks, Vol. 4, No. 4, 1997. This system allowsboth contention-based and contention-free access to the physical TDDchannel slots. This system also introduced the broadcast of resourceallocation at the start of every frame by the base station controller.Other wireless standards, including IEEE 802.16 wireless metropolitannetwork standards, use this combination of an allocation MAP of theuplink and downlink at the start of the dynamic TDD frame to setresource use for the next TDD frame.

[0041] An further improvement to this TDD system was described in“Multiple Access Control Protocols for Wireless ATM: Problem Definitionand Design Objectives,” O. Kubbar and H. Mouftah, IEEE Communications,November 1997, pp. 93-99. This system expanded on the packet reservationmultiple access (PRMA) method developed in 1989 at Rutgers UniversityWINLAB for ATM and IP based transport [see “Packet Reservation MultipleAccess for Local Wireless Communications,” Goodman et al., IEEETransaction on Communications, Vol. 37, No. 8, pp. 885-890]. Like PRMA,this system logically arranged all the downlink transmissions in thestart of a fixed duration TDD frame and all uplink transmissions at theend of the TDD frame. This eliminated the inefficiencies in the DCA++Hyperlan II protocol. Adaptive allocation of uplink and downlinkbandwidth is supported. The system provided for fixed, random, anddemand assignment mechanisms. Priority is given to quality of service(QoS) applications with resources being removed from best efforts demandaccess users as required.

[0042] The above-described prior art concern the allocation of servicesin an individual sector of a cell. A cell may consist of M sectors,wherein each sector generally covers a 360/M degree arc around the cellsite. Each sector serves N_(m) subscribers, where m=1 to M. Thesereferences did not expressly provide protocol mechanisms or rules forthe operation of a given system.

[0043] U.S. Pat. No. 6,016,311 expressly addresses one possibleimplementation to the TDD bandwidth allocation problem. The systemdescribed continuously measures and adapts the bandwidth requirementsbased on the evaluation of the average bandwidth required by all thesubscribers in a cell and the number of times bandwidth is denied to thesubscribers. Changes to the bandwidth allocation are applied based on aset of rules described in U.S. Pat. No. 6,016,311. While measurements ofmultiple sectors are performed and recorded at a central base stationcontroller, no global coordination of bandwidth allocation of multiplesectors in a cell or across multiple cells is provided.

[0044] Thus, the prior art does not address two very important factorsin allocation of bandwidth. First, bandwidth allocation must contemplatestringent bandwidth availability requirements for specific groups ofservices based on planning of the network. For example, considerlife-line toll quality voice service. Toll quality voice requires that asystem guarantee a specific maximum blocking probability for all voiceusers based on peak busy hour call usage. A description of voice trafficplanning is provided in “Digital Telephony—2^(nd) Edition,” by J.Bellamy, John Wiley and Sons, New York, New York, 1990. If a TDD systemis designed to meet life-line voice requirements, the allocationprotocol must be able to rapidly (i.e., less than 100 msec) reallocatebandwidth resources up to the capacity necessary to meet the callblocking requirements. Another service group example is a guaranteedservice level agreements (SLA). Again, bandwidth must be rapidlyrestored to meet the SLA conditions. More generally, one may consider Gpossible service groups having a set of weighted priority level andassociated minimum and maximum levels. The weighted priority levels andminimum and maximum levels may be used to bound the bandwidth dynamicsof the TDD bandwidth allocation. Minimum levels set a floor forbandwidth allocation and maximum levels set a ceiling. Then averagingcan be applied.

[0045] Second, the TDD bandwidth allocation must consider adjacent andco-channel interference from both modems and sectors within a cell andbetween cells. Cell planning tools can be used to establish therelationships for interference. For systems that operate below 10 GHz,antennas and antenna placement at a cell site will not provide adequatesignal isolation. These co-channel interference issues are welldocumented in “Frequency Reuse and System Deployment in Local MultipointDistribution Service,” by V. Roman, IEEE Personal Communications,December 1999, pp. 20 to 27.

[0046] Therefore, there is a need in the art for a fixed wireless accessnetwork that maximizes spectral efficiency between the base stations ofthe fixed wireless access network and the subscriber access deviceslocated at the subscriber premises. In particular, there is a need for afixed wireless access network that implements an air interface thatminimizes uplink and downlink interference between different sectorswithin the same base station cell site. There also is a need for a fixedwireless access network that implements an air interface that minimizesuplink and downlink interference between different cell sites within thefixed wireless access network. More particularly, there is a need in theart for a fixed wireless that efficiently allocates bandwidth toindividual subscribers according to dynamically changing applicationsused by the individual subscribers.

SUMMARY OF THE INVENTION

[0047] To address the above-discussed deficiencies of the prior art, itis a primary object of the present invention to provide an improved airinterface system for use in a fixed wireless access network thatmaximizes usage of the available bandwidth in a cell site. The systemuses multiple modulation groups in the air interface to transmit datato, and to receive data from, subscriber access devices in each cellsite and/or each sector within a cell site. Each subscriber accessdevice is added to a modulation group according to the modulationformat, FEC code, and/or antenna beam (spatial component) parameter thatgives the highest data throughput to each subscriber access devicewithin acceptable error limitations. Subscriber access devices thatreceive a comparatively noisy signal may receive a downlink signal inBPSK format with a high level of FEC protection. Subscriber accessdevices that receive a relatively noise-free signal may receive adownlink signal in 16 QAM format with a relatively low level of FECprotection. Subscriber access devices using the same modulation formatand FEC coding are grouped together in the same modulation groups in theuplink and the downlink. Furthermore, if two or more subscriber accessdevices lie in the same direction away from a base station, thosesubscriber access devices may also be grouped together in the samemodulation group according to antenna beam forming (i.e., spatialmodulation) parameters of the base station. Similar modulation groupingsare used in the uplink from the subscriber access devices to thetransceiver base station.

[0048] Thus, it is a primary object of the present invention to provide,a radio frequency (RF) modem shelf for use in a fixed wireless accessnetwork comprising a plurality of base stations capable of bidirectionaltime division duplex (TDD) communication with wireless access devicesdisposed at a plurality of subscriber premises. According to anadvantageous embodiment of the present invention, the radio frequency(RF) modem shelf comprises: a) a first RF modem capable of communicatingwith a plurality of the wireless access devices using TDD frames, eachTDD frame having an uplink for receiving data and a downlink fortransmitting data; b) a modulation controller associated with the RFmodem shelf capable of determining an optimum modulation configurationfor each of the plurality of wireless access devices communicating withthe first RF modem, wherein the modulation controller causes the firstRF modem to transmit downlink data to a first wireless access device ina first data block having a first optimum modulation configuration andto transmit downlink data to a second wireless access device in a seconddata block having a different second optimum modulation configuration.

[0049] According to one embodiment of the present invention, themodulation controller determines the first and second optimum modulationconfigurations based on channel conditions associated with channels usedto communicate with the first and second wireless access devices.

[0050] According to another embodiment of the present invention, thefirst modulation configuration comprises a first modulation format andthe second modulation configuration comprises a different secondmodulation format.

[0051] According to still another embodiment of the present invention,the second modulation format is more complex than the first modulationformat if channel conditions associated with a first channel used tocommunicate with the first wireless access device are noisier thanchannel conditions associated with a second channel used to communicatewith the second wireless access device.

[0052] According to yet another embodiment of the present invention, thefirst and second modulation formats comprise one of binary phase shiftkeying (BPSK), quadrature phase shift keying (QPSK), and 16 quadratureamplitude modulation (QAM).

[0053] According to a further embodiment of the present invention, thefirst modulation configuration comprises a first forward errorcorrection code level and the second modulation configuration comprisesa different second forward error correction code level.

[0054] According to a still further embodiment of the present invention,the first error correction code level is more complex than the seconderror correction code level if channel conditions associated with afirst channel used to communicate with the first wireless access deviceare noisier than channel conditions associated with a second channelused to communicate with the second wireless access device.

[0055] According to a yet further embodiment of the present invention,the first modulation configuration comprises a first physical beamforming technique and the second modulation configuration comprises adifferent second physical beam forming technique.

[0056] The foregoing has outlined rather broadly the features andtechnical advantages of the present invention so that those skilled inthe art may better understand the detailed description of the inventionthat follows. Additional features and advantages of the invention willbe described hereinafter that form the subject of the claims of theinvention. Those skilled in the art should appreciate that they mayreadily use the conception and the specific embodiment disclosed as abasis for modifying or designing other structures for carrying out thesame purposes of the present invention. Those skilled in the art shouldalso realize that such equivalent constructions do not depart from thespirit and scope of the invention in its broadest form.

[0057] Before undertaking the DETAILED DESCRIPTION OF THE INVENTIONbelow, it may be advantageous to set forth definitions of certain wordsand phrases used throughout this patent document: the terms “include”and “comprise,” as well as derivatives thereof, mean inclusion withoutlimitation; the term “or,” is inclusive, meaning and/or; the phrases“associated with” and “associated therewith,” as well as derivativesthereof, may mean to include, be included within, interconnect with,contain, be contained within, connect to or with, couple to or with, becommunicable with, cooperate with, interleave, juxtapose, be proximateto, be bound to or with, have, have a property of, or the like; and theterm “controller” means any device, system or part thereof that controlsat least one operation, such a device may be implemented in hardware,firmware or software, or some combination of at least two of the same.It should be noted that the functionality associated with any particularcontroller may be centralized or distributed, whether locally orremotely. Definitions for certain words and phrases are providedthroughout this patent document, those of ordinary skill in the artshould understand that in many, if not most instances, such definitionsapply to prior, as well as future uses of such defined words andphrases.

BRIEF DESCRIPTION OF THE DRAWINGS

[0058] For a more complete understanding of the present invention, andthe advantages thereof, reference is now made to the followingdescriptions taken in conjunction with the accompanying drawings,wherein like numbers designate like objects, and in which:

[0059]FIG. 1 illustrates an exemplary fixed wireless access networkaccording to one embodiment of the present invention;

[0060]FIG. 2 illustrates in greater detail an alternate view of selectedportions of the exemplary fixed wireless access network according to oneembodiment of the present invention;

[0061]FIG. 3 illustrates an exemplary time division duplex (TDD) timedivision multiple access (TDMA) frame according to one embodiment of thepresent invention;

[0062]FIG. 4 illustrates the timing recovery and distribution circuitryin an exemplary RF modem shelf according to one embodiment of thepresent invention;

[0063]FIG. 5A illustrates an exemplary time division duplex (TDD) framesaccording to one embodiment of the present invention;

[0064]FIG. 5B illustrates an exemplary transmission burst containing asingle FEC block according to one embodiment of the present invention;

[0065]FIG. 5C illustrates an exemplary transmission burst containingmultiple FEC blocks according to one embodiment of the presentinvention;

[0066]FIG. 6 is a flow diagram illustrating the adaptive modification ofthe uplink and downlink bandwidth in the air interface in wirelessaccess network according to one embodiment of the present invention;

[0067]FIG. 7 is a flow diagram illustrating the adaptive assignment ofselected link parameters, such as modulation format, forward errorcorrection (FEC) codes, and antenna beam forming, to the uplink anddownlink channels used by each subscriber in the exemplary wirelessaccess network according to one embodiment of the present invention; and

[0068]FIG. 8 is a flow diagram illustrating the adaptive assignment ofselected link parameters to the different service connections used byeach subscriber in the wireless access network according to oneembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0069]FIGS. 1 through 8, discussed below, and the various embodimentsused to describe the principles of the present invention in this patentdocument are by way of illustration only and should not be construed inany way to limit the scope of the invention. Those skilled in the artwill understand that the principles of the present invention may beimplemented in any suitably arranged wireless access system.

[0070]FIG. 1 illustrates exemplary fixed wireless access network 100according to one embodiment of the present invention. Fixed wirelessnetwork 100 comprises a plurality of transceiver base stations,including exemplary transceiver base station 110, that transmit forwardchannel (i.e., downlink or downstream) broadband signals to a pluralityof subscriber premises, including exemplary subscriber premises 121, 122and 123, and receive reverse channel (i.e., uplink or upstream)broadband signals from the plurality of subscriber premises. Subscriberpremises 121-123 transmit and receive via fixed, externally-mountedantennas 131-133, respectively. Subscriber premises 121-123 may comprisemany different types of residential and commercial buildings, includingsingle family homes, multi-tenant offices, small business enterprises(SBE), medium business enterprises (MBE), and so-called “SOHO” (smalloffice/home office) premises.

[0071] The transceiver base stations, including transceiver base station110, receive the forward channel (i.e., downlink) signals from externalnetwork 150 and transmit the reverse channel (i.e., uplink) signals toexternal network 150. External network 150 may be, for example, thepublic switched telephone network (PSTN) or one or more data networks,including the Internet or proprietary Internet protocol (IP) wide areanetworks (WANs) and local area networks (LANs). Exemplary transceiverbase station 110 is coupled to RF modem shelf 140, which, among otherthings, up-converts baseband data traffic received from external network150 to RF signals transmitted in the forward channel to subscriberpremises 121-123. RF modem shelf 140 also down-converts RF signalsreceived in the reverse channel from subscriber premises 121-123 tobaseband data traffic that is transmitted to external network 150.

[0072] RF modem shelf 140 comprises a plurality of RF modems capable ofmodulating (i.e., up-converting) the baseband data traffic anddemodulating (i.e., down-converting) the reverse channel RF signals. Inan exemplary embodiment of the present invention, each of thetransceiver base stations covers a cell site area that is divided into aplurality of sectors. In an advantageous embodiment of the presentinvention, each of the RF modems in RF modem shelf 140 may be assignedto modulate and demodulate signals in a particular sector of each cellsite. By way of example, the cell site associated with transceiver basestation 110 may be partitioned into six sectors and RF modem shelf 140may comprise six primary RF modems (and, optionally, a seventh spare RFmodem), each of which is assigned to one of the six sectors in the cellsite of transceiver base station 110. In another advantageous embodimentof the present invention, each RF modem in RF modem shelf 140 comprisestwo or more RF modem transceivers which may be assigned to at least oneof the sectors in the cell site. For example, the cell site associatedwith transceiver base station 110 may be partitioned into six sectorsand RF modem shelf 140 may comprise twelve RF transceivers that areassigned in pairs to each one of the six sectors. The RF modems in eachRF modem pair may alternate modulating and demodulating the downlink anduplink signals in each sector.

[0073] RF modem shelf 140 is located proximate transceiver base station110 in order to minimize RF losses in communication line 169. RF modemshelf 140 may receive the baseband data traffic from external network150 and transmit the baseband data traffic to external network 150 via anumber of different paths. In one embodiment of the present invention,RF modem shelf 140 may transmit baseband data traffic to, and receivebaseband data traffic from, external network 150 through central officefacility 160 via communication lines 166 and 167. In such an embodiment,communication line 167 may be a link in a publicly owned or privatelyowned backhaul network. In another embodiment of the present invention,RF modem shelf 140 may transmit baseband data traffic to, and receivebaseband data traffic from, external network 150 directly viacommunication line 168 thereby bypassing central office facility 160.

[0074] Central office facility 160 comprises access processor shelf 165.Access processor shelf 165 provides a termination of data traffic forone or more RF modem shelves, such as RF modem shelf 140. Accessprocessor shelf 165 also provides termination to the network switchedcircuit interfaces and/or data packet interfaces of external network150. One of the principal functions of access processor shelf 165 is toconcentrate data traffic as the data traffic is received from externalnetwork 150 and is transferred to RF modem shelf 140. Access processorshelf 165 provides data and traffic processing of the physical layerinterfaces, protocol conversion, protocol management, and programmablevoice and data compression.

[0075]FIG. 2 illustrates in greater detail an alternate view of selectedportions of exemplary fixed wireless access network 100 according to oneembodiment of the present invention. FIG. 2 depicts additionaltransceiver base stations, including exemplary transceiver base stations110A through 110F, central office facilities 160A and 160B, and remoteRF modem shelves 140A through 140D. Central office facilities 160A and160B comprise internal RF modems similar to RF modem shelves 140Athrough 140D. Transceiver base stations 110A, 110B, and 110C aredisposed in cells sites 201, 202, and 203, respectively. In theexemplary embodiment, cell sites 201-203 (shown in dotted lines) arepartitioned into four sectors each. In alternate embodiments, sites 201,202, and 203 may be partitioned into a different number of sectors, suchas six sectors, for example.

[0076] As in FIG. 1, RF modem shelves 140A-140D and the internal RFmodems of central office facilities 160A and 160B transmit baseband datatraffic to, and receive baseband data traffic from, access processors incentral office facilities 160A and 160B of the PSTN. RF modem shelves140A-140D and the internal RF modems of central office facilities 160Aand 160B also up-convert incoming baseband data traffic to RF signalstransmitted in the forward (downlink) channel to the subscriber premisesand down-convert incoming RF signals received in the reverse (uplink)channel to baseband data traffic that is transmitted via a backhaulnetwork to external network 150.

[0077] Baseband data traffic may be transmitted from remote RF modemshelves 140A-140D to central office facilities 160A and 160B by awireless backhaul network or by a wireline backhaul network, or both. Asshown in FIG. 2, baseband data traffic is carried between central officefacility 160A and remote RF modem 140A by a wireline backhaul network,namely wireline 161, which may be, for example, a DS3 line or one to NT1 lines. A local multipoint distribution service (LMDS) wirelessbackhaul network carries baseband data traffic between central officefacilities 160A and 160B and remote RF modem shelves 140B, 140C, and140D. In a LMDS wireless backhaul network, baseband data traffic beingsent to remote RF modem shelves 140B, 140C, and 140D is transmitted bymicrowave from microwave antennas mounted on transceiver base stations110A, 110C, and 110F to microwave antennas mounted on transceiver basestations 110B, 110D, and 110E. Baseband data traffic being sent fromremote RF modem shelves 140B, 140C, and 140D is transmitted by microwavein the reverse direction (i.e., from transceiver base stations 110B,110D, and 110E to transceiver base stations 110A, 110C, and 110F).

[0078] At each of transceiver base stations 110B, 110D, and 110E,downlink data traffic from central office facilities 160A and 160B isdown-converted from microwave frequencies to baseband signals beforebeing up-converted again for transmission to subscriber premises withineach cell site. Uplink data traffic received from the subscriberpremises is down-converted to baseband signals before being up-convertedto microwave frequencies for transmission back to central officefacilities 160A and 160B.

[0079] Generally, there is an asymmetry of data usage in the downlinkand the uplink. This asymmetry is typically greater than 4:1(downlink:uplink). Taking into account the factors of data asymmetry,channel propagation, and available spectrum, an advantageous embodimentof the present invention adopts a flexible approach in which thephysical (PHY) layer and the media access (MAC) layer are based on theuse of time division duplex (TDD) time division multiple access (TDMA).TDD operations share a single RF channel between a transceiver basestation and a subscriber premises and use a series of frames to allocateresources between each user uplink and downlink. A great advantage ofTDD operation is the ability to dynamically allocate the portions of aframe allocated between the downlink and the uplink. This results in anincreased efficiency of operation relative to frequency division duplex(FDD) techniques. TDD operations typically may achieve a forty to sixtypercent advantage in spectral efficiency over FDD operations undertypical conditions. Given the short duration of the transmit and receivetime slots relative to changes in the channel, TDD operations alsopermit open loop power control, switched diversity techniques, andfeedforward and cyclo-stationary equalization techniques that reducesystem cost and increase system throughput.

[0080] To aid with periodic functions in the system, TDD frames aregrouped into superframes (approximately 10 to 20 milliseconds) Thesuperframes are further grouped into hyperframes (approximately 250 to1000 milliseconds). This provides a coordinated timing reference tosubscriber integrated access devices in the system. FIG. 3 illustratesan exemplary time division duplex (TDD) time division multiple access(TDMA) framing hierarchy according to one embodiment of the presentinvention. At the highest level, the TDD-TDMA framing hierarchycomprises hyperframe 310, which is X milliseconds (msec.) in length(e.g., 250 msec.<X<1000 msec.). Hyperframe 310 comprises N superframes,including exemplary superframes 311-316. Each of superframes 311-316 is20 milliseconds in duration.

[0081] Superframe 313 is illustrated in greater detail. Superframe 313comprises ten (10) TDD frames, including exemplary TDD frames 321-324,which are labeled TDD Frame 0, TDD Frame 1, TDD Frame 2, and TDD Frame9, respectively. In the exemplary embodiment, each TDD frame is 2milliseconds in duration. A TDD transmission frame is based on a fixedperiod of time during which access to the channel is controlled by thetransceiver base station.

[0082] Exemplary TDD frame 321 is illustrated in greater detail. TDDframe 321 comprises a downlink portion (i.e, base station to subscribertransmission) and an uplink portion (i.e., subscriber to base stationtransmission). In particular, TDD frame 321 comprises:

[0083] Frame header 330—Frame header 330 is a broadcast message thatsynchronizes the start of frame and contains access control informationon how the remainder of TDD frame 321 is configured. The modulationformat of frame header 330 is chosen so that all subscribers in a sectorof the transceiver base station can receive frame header 330. Generally,this means that frame header 330 is transmitted in a very low complexitymodulation format, such as binary phase shift keying (BPSK or 2-BPSK),or perhaps quadrature phase shift keying (QPSK or 4-BPSK).

[0084] D downlink slots—The D downlink slots, including exemplarydownlink slots 341-343, contain transceiver base station-to-subscribertransmissions of user traffic and/or control signals. The modulationformat of each slot is optimized for maximum possible data transmissionrates. Downlink slots may be grouped in blocks to form modulation groupsas shown in FIG. 5A. Subscribers who receive data using the samemodulation format (or modulation index) and the same forward errorcorrection (FEC) codes are grouped together in the same modulationgroup. In some embodiment of the present invention, two or moremodulation groups may have the same modulation format and FEC codes. Inalternate embodiments of the present invention, downlink slots may begrouped in blocks based on physical beam forming, rather than onmodulation format and FEC codes. For example, a transceiver base stationmay transmit data to several subscribers that are directionally alongthe same antenna beam in consecutive bursts. In still other embodimentsof the present invention, downlink slots may be grouped in blocks basedon any combination of two or more of: 1) physical beam forming, 2)modulation format, and 3) FEC codes. For the purpose of simplicity, theterm “modulation group” shall be used hereafter to refer to a group ofdownlink slots that are transmitted to one or more subscribers using acommon scheme consisting of one or more of modulation format, FEC codes,and physical beam forming.

[0085] U uplink slots—The U uplink slots, including exemplary uplinkslots 361-363, contain subscriber-to-transceiver base stationtransmissions of user traffic and/or control signals. Again, themodulation format (modulation index) is optimized for maximum possibledata transmission rates. Generally, the modulation format and FEC codesin the uplink slots are less complex than in the downlink slots. Thismoves complexity to the receivers in the base stations and lowers thecost and complexity of the subscriber access device. Uplink slots may begrouped in blocks to form sub-burst groups as shown in FIG. 5A.Subscribers who transmit data using the same modulation format (ormodulation index) and the same forward error correction (FEC) codes aregrouped together in the same sub-burst group. In some embodiments of thepresent invention, two or more sub-burst groups may have the samemodulation format and FEC codes. In other embodiments of the presentinvention, uplink slots may be grouped in blocks based on physical beamforming, rather than on modulation format and FEC codes. In otherembodiments, uplink slots may be grouped in blocks based on anycombination of two or more of: 1) physical beam forming, 2) modulationformat, and 3) FEC codes. For the purpose of simplicity, the term“sub-burst group” shall be used hereafter to refer to a group of uplinkslots that are transmitted to one or more subscribers using a commonscheme consisting of one or more of modulation format, FEC codes, andphysical beam forming.

[0086] Contention slots 360—Contention slots 360 precede the U uplinkslots and comprise a small number of subscriber-to-base transmissionsthat handle initial requests for service. A fixed format length and asingle modulation format suitable for all subscriber access devices areused during contention slots 360. Generally, this means that contentionslots 360 are transmitted in a very low complexity modulation format,such as binary phase shift keying (BPSK or 2-BPSK), or perhapsquadrature phase shift keying (QPSK or 4-BPSK). Collisions (more thanone user on a time slot) result in the use of back-off proceduressimilar to CSMA/CD (Ethernet) in order to reschedule a request.

[0087] TDD transition period 350—TDD transition period 350 separates theuplink portion and the downlink portion and allows for transmitter (TX)to receiver (RX) propagation delays for the maximum range of the celllink and for delay associated with switching hardware operations from TXto RX or from RX to TX. The position of TDD transition period 350 may beadjusted, thereby modifying the relative sizes of the uplink portion andthe downlink portion to accommodate the asymmetry between data trafficin the uplink and the downlink.

[0088] A key aspect of the present invention is that the timing of thedownlink and uplink portions of each TDD frame must be precisely alignedin order to avoid interference between sectors within the same celland/or to avoid interference between cells. It is recalled from abovethat each sector of a cell site is served by an individual RF modem inRF modem shelves 140A-140D and the internal RF modem shelves of centraloffice facilities 160A and 160B. Each RF modem uses an individualantenna to transmit and to receive in its assigned sector. The antennasfor different sectors in the same cell site are mounted on the sametower and are located only a few feet apart. If one RF modem (andantenna) are transmitting in the downlink while another RF modem (andantenna) are receiving in the uplink, the power of the downlinktransmission will overwhelm the downlink receiver.

[0089] Thus, to prevent interference between antennas in differentsectors of the same cell site, the present invention uses a highlyaccurate distributed timing architecture to align the start points ofthe downlink transmissions. The present invention also determines thelength of the longest downlink transmission and ensures that none of theuplink transmissions begin, and none of the base station receivers beginto receive, until after the longest downlink is completed.

[0090] Furthermore, the above-described interference between uplink anddownlink portions of TDD frames can also occur between different cellsites. To prevent interference between antennas in different cell sites,the present invention also uses the highly accurate distributed timingarchitecture to align the start points of the downlink transmissionsbetween cell sites. The present invention also determines the length ofthe longest downlink transmission among two or more cell sites andensures that none of the base station receivers in any of the cellsbegins to receive in the uplink until after the longest downlinktransmission is completed.

[0091] Within a cell site, a master interface control processor (ICP),as described below in FIG. 4, may be used to align and allocate theuplink and downlink portions of the TDD frames for all of the RF modemsin an RF modem shelf. Between cell sites, the access processor maycommunicate with several master ICPs to determine the longest downlink.The access processor may then allocated the uplinks and downlinks acrossseveral cell sites in order to minimize interference between cell sitesand may designate on master ICP to control the timing of all of themaster ICPs.

[0092]FIG. 4 illustrates the timing recovery and distribution circuitryin exemplary RF modem shelf 140 according to one embodiment of thepresent invention. RF modem shelf 140 comprises front panel interface410 having connectors 411-414 for receiving input clock references andtransmitting clock references. Exemplary connector 411 receives a firstclock signal from a first external source (External Source A) andexemplary connector 414 receives a second clock signal from a secondexternal source (External Source B). Connector 412 outputs an internallygenerated clock signal (Master Source Out) and connector 413 receives anexternal one second system clock signal (External 1 Second Clock).

[0093] RF modem shelf 140 also comprises a plurality of interfacecontrol processor (ICP) cards, including exemplary ICP cards 450, 460,470 and 480. ISP card 450 is designated as a master ICP card and ICPcard 480 is designated as a spare ICP card in case of a failure ofmaster ICP card 450. Within RF modem shelf 140, the ICP cards providefor control functions, timing recovery and distribution, networkinterface, backhaul network interface, protocol conversion, resourcequeue management, and a proxy manager for EMS for the shelf. The ICPcards are based on network processor(s) that allow software upgrade ofnetwork interface protocols. The ICP cards may be reused for control androuting functions and provide both timing and critical TDD coordinatedburst timing for all the RF modems in RF modem shelf 140 and forshelf-to-shelf timing for stacked frequency high density cellconfigurations.

[0094] The timing and distribution architecture in RF modem shelf 140allows for three reference options:

[0095] Primary—An external input derived from another remote modem shelfacting as a master. BITS (Building Integrated Timing Supply) referenceis a single building master timing reference (e.g., External Source A,External Source B) that supplies DS1 and DS0 level timing throughout anoffice (e.g., 64K or 1.544/2.048 Mbps).

[0096] Secondary—A secondary reference may be derived from anydesignated input port in RF modem shelf 140. For remote RF modem shelf140, this is one of the backhaul I/O ports. An ICP card is configured torecover a timing source and that source is placed on a backplane as areference (i.e., Network Reference (A/B)) to master ICP card 450.

[0097] Tertiary—An internal phase locked loop (PLL) may be used.

[0098] By default, two ICP cards are configured as a master ICP card anda spare ICP card. The active master ICP card distributes timing for allof RF modem shelf 140. The timing distribution architecture of RF modemshelf 140 meets Stratum 3 levels of performance, namely a free-runaccuracy of +/−4.6 PPM (parts per million), a pull-in capability of 4.6PPM, and a holdover stability of less than 255 slips during the firstday.

[0099] There are three components to the timing distribution for theaccess processor backplane:

[0100] 1. Timing masters (ICP cards 450 and 480).

[0101] 2. Timing slaves (ICP cards 460 and 470).

[0102] 3. Timing references.

[0103] The timing masters are capable of sourcing all clocks and framingsignals necessary for the remaining cards within the AP backplane.Within a backplane, there are two timing masters (ICP cards 450 and480), which are constrained to the slots allocated as the primary andsecondary controllers. The timing masters utilize the redundant timingreferences (External Source A, External Source B, External 1 SecondClock) found on the backplane to maintain network-qualifiedsynchronization. ISP card 450 (and ISP card 480) comprises backhaulnetwork input/output (I/O) port 451, multiplexer 452 and PLL-clockgenerator 453. MUX 452 selects anyone of External Source A, ExternalSource B, Network Reference (A/B), and the signal from I/O port 451 tobe applied to PLL-clock generator 453. The timing master has missingclock detection logic that allows it to switch from one timing referenceto another in the event of a failure.

[0104] Timing is distributed across a redundant set of clock and framingsignals, designated Master Clock Bus in FIG. 4. Each timing master(i.e., ICP cards 450 and 480) is capable under software control ofdriving either of the two sets of clock and framing buses on thebackplane. Both sets of timing buses are edge-synchronous such thattiming slaves can interoperate while using either set of clocks.

[0105] The timing supplied by the timing master (e.g., ICP card 450)consists of a 65.536 MHZ clock and an 8 KHz framing reference. There isa primary and secondary version of each reference. To generate thesereferences, the primary and secondary timing masters are provisioned torecover the timing from one of the following sources: Source FrequencyExternal BITS (EXT REF A) 64 K, 1544 K, 2048 K External BITS/GPS (EXTREF B) 64 K, 1544 K, 2048 K External GPS Sync. Pulse 1 second pulseOn-Card Reference Per I/O reference Network I/O Derived Reference A PerI/O reference Network I/O Derived Reference B Per I/O reference

Table of Clock Source Interface Definitions

[0106] To simplify clock distribution and to provide redundancy all theclocks are derived from a common clock source. The following tablesummarizes the backplane reference clocks as well as the clock rates ofthe various backplane resources and how they are derived from thesereferences. Clock Frequency Division or Ratio Common 65.536 MHZ NotApplicable Reference Clock Common Sync 1 Hz Not Applicable Pulse Framing8 KHz (125 usec) Free-run framing provided Reference by Primary orSecondary Clock Masters Referenced to Common Reference Clock Cell/Packet32.768 MHZ Reference Clock/2 Clock Rate TDM Bus Rate 8.192 MHZ ReferenceClock/8 RF Reference 10.000 MHZ Free-run RF reference clock ClockCommunications 100 MHZ Derived from free-run Bus Reference ClockHigh-speed 1.31072 GHz REF. Clock × 20 Serial Links High-speed 2.62144GHz REF. Clock × 40 Serial Links High-speed TBD REF. Clock × N SerialLinks

Table of Buses and Associated Clocks

[0107] Timing slaves (i.e., ICP cards 460 and 470) receive the timingprovided by redundant sets of clock and framing buses. Under softwarecontrol, timing slaves choose a default set of clocks from either theA-side or B-side timing buses. They also contain failure detection logicsuch that clock and framing signal failures can be detected. Once aclock or framing failure is detected, the timing slave automaticallyswitches to the alternate set of timing buses. ICP cards 460 and 470contain backhaul I/O ports 461 and 471, respectively, which may be usedto bring in external timing signals from other RF modem shelves in thenetwork. The timing masters (i.e., ICP cards 450 and 480) also containthe timing slave function insofar as they also utilize the timingprovided on the backplane clock and framing buses.

[0108] A qualified timing reference is required for the timing master toderive backplane timing and to maintain synchronization within network100 and with any outside network. Under software control, an accessprocessor card can be assigned to derive this timing and to drive one ofthe two timing reference buses. Ideally, a second, physically separatecard will contain a second qualified timing source and drive the secondbackplane timing reference.

[0109] In the event that no qualified timing is present from trunkinterfaces, the access processor backplane has connections which allowexternal reference timing (e.g., a GPS-derived clock) from the interfacetray to be applied to the backplane. A one pulse-per-second (1PPS)signal is distributed to all system cards for time stamping of systemevents and errors. Installations involving multiple access processorshelves require the timing reference to be distributed between allaccess processor backplanes. In this scenario, the timing reference fora given backplane is cabled to the remaining backplanes through externalcabling. Multiple remote modem shelves are utilized to distributehigh-capacity backhaul traffic to one or more additional co-locatedmodem shelves. Traffic is distributed among the shelves through T1, T3,OC3 and/or other broadband telecommunication circuits. To maintainnetwork timing, the additional shelves are slaved to these distributionlinks and recover timing through the same PLL mechanisms as the head-endshelf.

[0110]FIG. 5A illustrates exemplary time division duplex (TDD) frame 500according to one embodiment of the present invention. FIG. 5Billustrates exemplary transmission burst 520 containing a single FECblock according to one embodiment of the present invention. FIG. 5Cillustrates exemplary transmission burst 530 containing multiple FECblocks according to one embodiment of the present invention.

[0111] TDD frame 500 comprises a downlink portion containing preamblefield 501, management field 502, and N modulation groups, includingmodulation group 503 (labeled Modulation Group 1), modulation group 504(labeled Modulation Group 2), and modulation group 505 (labeledModulation Group N). As explained above in FIG. 3, a modulation group isa group of downlink slots transmitted to one or more subscribers using acommon scheme of one or more of: 1) modulation format, 2) FEC codes, and3) physical beam forming.

[0112] TDD frame 500 also comprises an uplink portion containingtransmitter-transmitter guard (TTG) slot 506, 0 to N registration (REG)minislots 506, 1 to N contention (CON) request minislots 508, Nsub-burst groups, including sub-burst group 509 (labeled Sub-Burst 1)and sub-burst group 510 (labeled Sub-Burst N), and receiver-transmitterguard (RTG) slot 511. As explained above in FIG. 3, a sub-burst group isa group of uplink slots transmitted to one or more subscribers using acommon scheme of one or more of: 1) modulation format, 2) FEC codes, and3) physical beam forming.

[0113] Each modulation group and each sub-burst group comprises one ormore transmission bursts. Exemplary transmission burst 520 may be usedwithin a single modulation group in the downlink and covers one or moredownlink slots. Transmission burst 520 also may be used within a singlesub-burst group in the downlink and covers one or more uplink slots.Transmission burst 520 comprises physical media dependent (PMD) preamblefield 521, MAC header field 522, data packet data unit (PDU) field 523,and block character redundancy check (CRC) field 524. Transmission burst530 comprises physical media dependent (PMD) preamble field 531, MACheader field 532, data PDU field 533, block CRC field 534, data PDUfield 535, block CRC field 536.

[0114] The start of every frame includes a Start-Of-Frame (SOF) fieldand a PHY Media Dependent Convergence (PMD) field. PMD preambles areused to assist in synchronization and time-frequency recovery at thereceiver. The SOF field allows subscribers using fixed diversity to testreception conditions of the two diversity antennas.

[0115] The SOF PMD field is 2^(N) symbols long (typically 16, 32, 64symbols long) and consists of pseudo-random noise (PN) code sequences,Frank sequences, CAZAC sequences, or other low cross-correlationsequences, that are transmitted using BPSK or QPSK modulation. The SOFfield is followed by downlink management messages broadcast from thebase station to all subscribers using the lowest modulation or FEC indexand orthogonal expansion. Management messages are transmitted bothperiodically (N times per hyperframe) and as required to changeparameters or allocate parameters. Management messages include:

[0116] 1. DownLink Map indicating the physical slot (PS) wheredownstream modulation changes (transmitted every frame);

[0117] 2. UpLink MAP indicating uplink subscriber access grants andassociated physical slot start of the grant (transmitted when changedand at a minimum of one second hyperframe periods (shorter periods areoptional));

[0118] 3. TDD frame and physical layer attributes (periodic at a minimumof one second hyperframe period); and

[0119] 4. Other management messages including ACK, NACK, ARQ requests,and the like (transmitted as required).

[0120] The downlink management messages are followed by multi-cast anduni-cast bursts arranged in increasing modulation complexity order. Thepresent invention introduces the term “modulation group” to define a setof downstream bursts with the same modulation and FEC protection. Asubscriber continuously receives all the downstream data in the TDDframe downlink until the last symbol of the highest modulation groupsupported by the link is received. This allows a subscriber maximum timeto perform receive demodulation updates.

[0121] The downlink-to-uplink transition provides a guard time (TTG) toallow for propagation delays for all the subscribers. The TTG positionand duration is fully programmable and set by management physical layerattribute messages. The TTG is followed by a set of allocated contentionslots that are subdivided between acquisition uplink ranging mini-slotsand demand access request mini-slots. The Uplink MAP message establishesthe number and location of each type of slot. Ranging slots are used forboth initial uplink synchronization of subscribers performing net entryand for periodic update of synchronization of active subscribers.Contention slots provide a demand access request mechanism to establishsubscriber service for a single traffic service flow. As collisions arepossible, the subscriber uses random back-off, in integer TDD frameperiods and retries based on a time out for request of service.Contention slots use the lowest possible modulation, FEC, and orthogonalexpansion supported by the base station.

[0122] The contention slots are followed by individual subscribertransmissions (sub-bursts) that have been scheduled and allocated by thebase station in the uplink MAP. Each subscriber transmission burst isperformed at the maximum modulation, FEC, and orthogonal expansionsupported by the subscriber. Finally, the subscriber transmissions arefollowed by the uplink-to-downlink transition which provides a guardtime (RTG) to allow for propagation delays for all the subscribers. TheRTG duration is fully programmable and set by management physical layerattribute messages.

[0123] In the downlink, the Physical Media Dependent (PMD) burstsynchronization is not used. The transmission burst begins with the MACheader and is followed by the packet data unit (PDU) and the associatedblock CRC field that protects both the PDU and the header. The PDU maybe a complete packet transmission or a fragment of a much largermessage. When a channel requires more robust FEC, the PDU may be brokeninto segments that are protected by separate FEC CRC fields. This avoidswasting bandwidth with additional MAC headers.

[0124] One significant difference between the uplink and the downlink isthe addition of the PMD preamble. The PMD preamble length and patterncan be programmed by transceiver base station 110. Like the SOF field atthe beginning of the TDD Frame, the preamble provides a synchronizationmethod for the base station receiver. Uplink registration and rangingpacket bursts use longer PMD preambles.

[0125] The medium access control (MAC) layer protocol is connectionoriented and provides multiple connections of different quality ofservice (QoS) to each subscriber. The connections are established when asubscriber is installed and enters operation fixed wireless accessnetwork 100. Additional connections can be established and terminatedwith the base station transceivers as subscriber requirements changes.

[0126] As an example, suppose a subscriber access device supports twovoice channels and a data channel. The quality of service (QoS) on bothof the voice channels and data can set based on the service structureset by the wireless service provider. At installation, a subscriber maystart with two service connections: a toll quality voice channel and amedium data rate broadband data connection. At a later point in time,the subscriber may order and upgrade service to two toll quality voicechannels and high speed data connection (a total of three connections).

[0127] The maintenance of connections varies based on the type ofconnection established. T1 or fractional T1 service requires almost nomaintenance due to the periodic nature of transmissions. A TCP/IPconnection often experiences bursty on-demand communication that may beidle for long periods of time. During those idle periods, periodicranging and synchronization of the subscriber is required.

[0128] In an exemplary embodiment of fixed wireless access network 100,each subscriber maintains a 64-bit EUI for globally unique addressingpurposes. This address uniquely defines the subscriber from within theset of all possible vendors and equipment types. This address is usedduring the registration process to establish the appropriate connectionsfor a subscriber. It is also used as part of the authentication processby which the transceiver base station and the subscriber each verify theidentity of the other.

[0129] In the exemplary embodiment, a connection may be identified by a16-bit connection identifier (CID) in MAC header 522 or MAC header 532.Every subscriber must establish at least two connections in eachdirection (upstream and downstream) to enable communication with thebase station. The basic CIDs, assigned to a subscriber at registration,are used by the base station MAC layer and the subscriber MAC layer toexchange MAC control messages, provisioning and management information.

[0130] The connection ID can be considered a connection identifier evenfor nominally connectionless traffic like IP, since it serves as apointer to destination and context information. The use of a 16-bit CIDpermits a total of 64K connections within the sector.

[0131] In an exemplary embodiment of fixed wireless access network 100,the CID may be divided into 2 fields. Bits [16:x] may be used touniquely identify a subscriber. In a cyclo-stationary receiverprocessing at a base station, this would set the antenna, equalizer, andother receiver parameters. Bits [x:1] may be used to indicate aconnection to a type of service. Each subscriber service can haveindividual modulation format, FEC, and ARQ. Thus, within a singlesub-burst group transmitted by a subscriber, the voice data may use onetype of modulation format, FEC, and ARQ, and the broadband internetservice may use a different modulation format, FEC, and ARQ. Similarly,within a single modulation group transmitted to the subscriber, thevoice data may use one type of modulation format, FEC, and ARQ, and thebroadband internet service may use a different modulation format, FEC,and ARQ.

[0132] As an example, bits [16:7] of the CID may identify 2^ 10 (or1024) distinct subscribers and bits [6:1] may identify 2^ 6=64 possibleconnections. An apartment building could be given a set of subscriberports [16:9] SO that bits [9:7] allow 2^ 8 connections or 256connections.

[0133] Requests for transmission are based on these connection IDs,since the allowable bandwidth may differ for different connections, evenwithin the same service type. For example, a subscriber unit servingmultiple tenants in an office building would make requests on behalf ofall of them, though the contractual service limits and other connectionparameters may be different for each of them.

[0134] Many higher-layer sessions may operate over the same wirelessconnection ID. For example, many users within a company may becommunicating with TCP/IP to different destinations, but since they alloperate within the same overall service parameters, all of their trafficis pooled for request/grant purposes. Since the original LAN source anddestination addresses are encapsulated in the payload portion of thetransmission, there is no problem in identifying different usersessions.

[0135] Fragmentation is the process by which a portion of a subscriberpayload (uplink or downlink) is divided into two or more PDUs.Fragmentation allows efficient use of available bandwidth whilemaintaining the QoS requirements of one or more of services used by thesubscriber. Fragmentation may be initiated by a base station for adownlink connection or the subscriber access device for the uplinkconnection. A connection may be in only one fragmentation state at anygiven time. The authority to fragment data traffic on a connection isdefined when the connection is created.

[0136] The MAC layer protocol in wireless access network 100 alsosupports concatenation of multiple PDUs in a single transmission in boththe uplink and the downlink, as shown in FIG. 5C. Since each PDUcontains a MAC header with the CID, the receiving MAC layer candetermine routing and processing by higher layer protocols. A basestation MAC layer creates concatenated PDUs in the uplink MAP.Management, traffic data, and bandwidth may all be concatenated. Thisprocess occurs naturally in the downlink. In the uplink, concatenationhas the added benefit of eliminating additional PMD preambles.

[0137]FIG. 6 depicts flow diagram 600, which illustrates the adaptivemodification of the uplink and downlink bandwidth in the air interfacein wireless access network 100 according to one embodiment of thepresent invention. Initially, an RF modem shelf, such as RF modem shelf140A, receives new access requests from subscriber access devices infixed wireless access network 100 and determines traffic requirementsfor each new and existing subscriber in each sector of a single cellsite (process step 605). The traffic requirements of each subscriber maybe established in a number of ways, including minimum QoS requirements,service level agreements, past usage, and current physical layerparameters, such as modulation index, FEC codes, antenna beam forming,and the like. The RF modem shelf then determines from the subscribertraffic requirements the longest downlink portion of any TDD frame ineach sector of a single cell site (process step 610).

[0138] Next, the access processor for the RF modem shelf (or the RFmodem shelf itself) determines the appropriate allocation of downlinkand uplink portions of TDD frames for a single cell site in order tominimize or eliminate interference within the cell site (process step615). Bandwidth is allocated, and TDD transition period 350 ispositioned, such that the longest downlink transmission is completebefore any receiver in the cell site starts to listen for the uplinktransmission.

[0139] Next, if global allocation of downlink and uplink bandwidthacross multiple cell sites is being implemented (generally the case),the access processor determines the longest downlink portion of any TDDframe across several closely located cell sites stations (process step620). The access processor then determines the allocation of uplink anddownlink bandwidth for all TDD frames across several closely locatedcell sites in order to minimize or eliminate cell-to-cell interference(process step 625). Again, bandwidth is allocated, and TDD transitionperiod 350 is positioned, such that the longest downlink transmission iscomplete before any receiver in any of the closely located cell sitesstarts to listen for uplink transmissions. Finally, the downlinkportions of the TDD frames are launched simultaneously using the highlyaccurate clock from the distributed timing architecture (process step630).

[0140] The dynamic application of TDD bandwidth allocation is bounded byset minimum and maximum boundaries set by the service provider, based ontraffic and network analysis. Further, the bandwidth bounds may beallocated in sub-groupings based on established quality of service (QoS)requirements (e.g., voice data) and Service Level Agreements (SLA)(e.g., broadband data rate) as the primary consideration and with bestefforts, non-QoS data, and IP traffic as secondary considerations. Thebandwidth bounds may be allocated based on the fact that a subscribermay support more that one interface and thus more than one modulationformat in order to achieve required error rates for one or more servicesprovided to the subscriber.

[0141]FIG. 7 depicts flow diagram 700, which illustrates the adaptiveassignment of selected link parameters, such as modulation format,forward error correction (FEC) codes, and antenna beam forming, to theuplink and downlink channels used by each subscriber in wireless accessnetwork 100 according to one embodiment of the present invention. The RFmodem shelf monitors data traffic between subscribers and base stationand determines for each subscriber the most efficient combination ofmodulation format, FEC code, and/or antenna beam forming for the uplinkand downlink.

[0142] The selected combination is based at least in part on the errorrates detected by the RF modem shelf when monitoring the data traffic.If the error rate for a particular subscriber is too high in either theuplink or the downlink, the RF modem shelf can decrease the modulationformat complexity and use a higher level of FEC code protection ineither the uplink or the downlink in order to reduce the error rate.Conversely, if the error rate for a particular subscriber is very low ineither the uplink or the downlink, the RF modem shelf can increase themodulation format complexity and use a lower level of FEC codeprotection in either the uplink or the downlink in order to increase thespectral efficiency, provided the error rate remains acceptably low.Different modulation formats and FEC codes may be used for differentservices (e.g., voice, data) used by a subscriber (process step 705).

[0143] Next, the RF modem shelf assigns subscribers to modulation groupsin the downlink and to sub-burst groups in the uplink (process step710). The base station transceiver then transmits media access fields(e.g., signaling, ACK & NACK) using the lowest modulation format/FECcode complexity. The base station transceiver then transmits theremaining modulation groups in the downlink to the subscribers inincreasing order of modulation format/FEC code complexity (process step715). When the downlink is complete, the base station transceiverreceives registration & contention minislots transmitted by thesubscriber access devices using the lowest modulation format/FEC codecomplexity. The base station transceiver then receives the remainingsub-burst groups transmitted by the subscribers in increasing order ofmodulation format/FEC code complexity (process step 720).

[0144] The use of adaptive link parameters improves the link throughputand correspondingly affects the bandwidth allocation described above inFIG. 6. Link parameters apply not only to the transmitter but to thereceiver as well. The present invention uses a bounded (finite) set ofmodulation formats to maximize bandwidth utilization to each subscriberin a channel or sector. In an exemplary embodiment of the presentinvention, the low complexity (low bandwidth efficiency) modulationformats used for media access fields (e.g., signaling, ACK, NACK) arebinary phase shift keying (BSPK or 2-PSK) and quadrature phase shiftkeying (QPSK or 4-PSK). The present invention may also use multiple-codeorthogonal expansion codes in conjunction with the low complexitymodulation formats for extremely robust communication. The highercomplexity (higher efficiency) modulation formats used for themodulation groups and the sub-burst groups may be 8-PSK, 16 quadratureamplitude modulation (QAM), 32 QAM, 64 QAM, 128 QAM, and the like.

[0145] The present invention also uses bounded set of FEC codes tomaximize bandwidth utilization to each subscriber in a channel orsector. The level of FEC code protection is based on the servicesprovided. Each subscriber may support multiple services.

[0146] In an advantageous embodiment of the present invention, the RFmodem shelf may use packet fragmentation to transport data in either theuplink or the downlink. Fragmentation is the division of larger packetsinto smaller packets (fragments) combined with an ARQ (automatic requestfor retransmission) mechanism to retransmit and recover erroneousfragments. The RF modem shelf automatically reduces fragment size forhigh error rate channels. Fragmentation is applied for guaranteederror-free sources. The degree of fragmentation and ARQ is based on theservice provided, since each subscriber may support multiple services.

[0147]FIG. 8 depicts flow diagram 800, which illustrates the adaptiveassignment of selected link parameters to the different serviceconnections used by each subscriber in wireless access network 100according to one embodiment of the present invention. The RF modem shelfassigns connection identification (CID) values to the uplink and to theuplink connections used by a subscriber. If a subscriber uses more thanone service (e.g., two voice, One data), the RF modem shelf assignsseparate CID values to each uplink connection and separate CID values toeach downlink connection (process step 805). As noted above, the CIDcomprises a bit field with the uppermost bits identifying the subscriberand the lowermost bits identifying a specific connection to thesubscriber. While many sets of adaptive transmission and receptionparameters are possible, there are a finite number of combination thatmake logical sense. These combinations are grouped into physical layerusage codes that are broadcast to subscribers as part of the generalheader of TDD superframe or frame header on a periodic basis. Theseapply to both the base station transmissions and the subscribertransmissions.

[0148] The RF modem shelf monitors data traffic between subscriber andbase station and determines for each connection the most efficientcombination of modulation format, FEC code, and/or antenna beam formingfor the uplink and downlink (process step 810). The RF modem shelf thenassigns each subscriber connection to a modulation group in the downlinkand to a sub-burst group in the uplink (process step 815). The basestation transmits media access fields (e.g., signaling, ACK & NACK)using the lowest modulation format/FEC code complexity. Then basestation then transmits modulation groups to subscribers in increasingorder of modulation format/FEC code complexity (process step 820).Finally, the base station receives registration & contention minislotsusing the lowest modulation format/FEC code complexity. Then basestation then receives sub-burst groups from subscribers in increasingorder of modulation format/FEC code complexity (process step 825).

[0149] Physical layer usage codes are bound to subscriber CID values bya service establishment protocol. If there is a degradation orimprovement in the channel between a subscriber and the base station, aprotocol exists so the subscriber access device and the base station mayrevise the physical layer usage code and subscriber CID code. The codesand bindings can be added and deleted based on services requirements ofthe subscriber.

[0150] Although the present invention has been described in detail,those skilled in the art should understand that they can make variouschanges, substitutions and alterations herein without departing from thespirit and scope of the invention in its broadest form.

What is claimed is:
 1. For use in a fixed wireless access networkcomprising a plurality of base stations capable of bidirectional timedivision duplex (TDD) communication with wireless access devicesdisposed at a plurality of subscriber premises, a radio frequency (RF)modem shelf comprising: a first RF modem capable of communicating with aplurality of said wireless access devices using TDD frames, each TDDframe having an uplink for receiving data and a downlink fortransmitting data; and a modulation controller associated with said RFmodem shelf capable of determining an optimum modulation configurationfor each of said plurality of wireless access devices communicating withsaid first RF modem, wherein said modulation controller causes saidfirst RF modem to transmit downlink data to a first wireless accessdevice in a first data block having a first optimum modulationconfiguration and to transmit downlink data to a second wireless accessdevice in a second data block having a different second optimummodulation configuration.
 2. The RF modem shelf as set forth in claim 1wherein said modulation controller determines said first and secondoptimum modulation configurations based on channel conditions associatedwith channels used to communicate with said first and second wirelessaccess devices.
 3. The RF modem shelf as set forth in claim 2 whereinsaid first modulation configuration comprises a first modulation formatand said second modulation configuration comprises a different secondmodulation format.
 4. The RF modem shelf as set forth in claim 3 whereinsaid second modulation format is more complex than said first modulationformat if channel conditions associated with a first channel used tocommunicate with said first wireless access device are noisier thanchannel conditions associated with a second channel used to communicatewith said second wireless access device.
 5. The RF modem shelf as setforth in claim 4 wherein said first and second modulation formatscomprise one of binary phase shift keying (BPSK), quadrature phase shiftkeying (QPSK), and 16 quadrature amplitude modulation (QAM).
 6. The RFmodem shelf as set forth in claim 3 wherein said first modulationconfiguration comprises a first forward error correction code level andsaid second modulation configuration comprises a different secondforward error correction code level.
 7. The RF modem shelf as set forthin claim 6 wherein said first error correction code level is morecomplex than said second error correction code level if channelconditions associated with a first channel used to communicate with saidfirst wireless access device are noisier than channel conditionsassociated with a second channel used to communicate with said secondwireless access device.
 8. The RF modem shelf as set forth in claim 2wherein said first modulation configuration comprises a first physicalbeam forming technique and said second modulation configurationcomprises a different second physical beam forming technique.
 9. A fixedwireless access network comprising: a plurality of base stations capableof bidirectional time division duplex (TDD) communication with wirelessaccess devices disposed at a plurality of subscriber premises; and aradio frequency (RF) modem shelf comprising: a first RF modem capable ofcommunicating with a plurality of said wireless access devices using TDDframes, each TDD frame having an uplink for receiving data and adownlink for transmitting data; and a modulation controller associatedwith said RF modem shelf capable of determining an optimum modulationconfiguration for each of said plurality of wireless access devicescommunicating with said first RF modem, wherein said modulationcontroller causes said first RF modem to transmit downlink data to afirst wireless access device in a first data block having a firstoptimum modulation configuration and to transmit downlink data to asecond wireless access device in a second data block having a differentsecond optimum modulation configuration.
 10. The fixed wireless accessnetwork as set forth in claim 9 wherein said modulation controllerdetermines said first and second optimum modulation configurations basedon channel conditions associated with channels used to communicate withsaid first and second wireless access devices.
 11. The fixed wirelessaccess network as set forth in claim 10 wherein said first modulationconfiguration comprises a first modulation format and said secondmodulation configuration comprises a different second modulation format.12. The fixed wireless access network as set forth in claim 11 whereinsaid second modulation format is more complex than said first modulationformat if channel conditions associated with a first channel used tocommunicate with said first wireless access device are noisier thanchannel conditions associated with a second channel used to communicatewith said second wireless access device.
 13. The fixed wireless accessnetwork as set forth in claim 12 wherein said first and secondmodulation formats comprise one of binary phase shift keying (BPSK),quadrature phase shift keying (QPSK), and 16 quadrature amplitudemodulation (QAM).
 14. The fixed wireless access network as set forth inclaim 10 wherein said first modulation configuration comprises a firstforward error correction code level and said second modulationconfiguration comprises a different second forward error correction codelevel.
 15. The fixed wireless access network as set forth in claim 14wherein said first error correction code level is more complex than saidsecond error correction code level if channel conditions associated witha first channel used to communicate with said first wireless accessdevice are noisier than channel conditions associated with a secondchannel used to communicate with said second wireless access device. 16.The fixed wireless access network set forth in claim 10 wherein saidfirst modulation configuration comprises a first physical beam formingtechnique and said second modulation configuration comprises a differentsecond physical beam forming technique.